Fig. 6.1 shows a capacitor of capacitance C connected in series with a resistor of resistance R. Initially the switch is open and there is a p.d. of 12 V across the capacitor. At time t = 0, the switch is closed so that there is a current I in the resistor. Fig. 6.2 shows the variation of I with t.

Explain the shape of the line in Fig. 6.2.